Traditionally, the referee or referees of a sports match decides from visual observation whether or not the ball has passed the goal plane. However, this may be very difficult to determine correctly in situations where the ball is returned quickly and has only just passed, or not passed, the goal plane, and it is particularly difficult if the referee is positioned unsuitably with respect to the goal plane or is engaged in other activity of the match. Video camera may also be used to monitor the goal planes, but the spatial and temporal resolution of video-cameras are often not sufficient to provide the necessary information in cases of doubt.
A number of electronic systems are known in the art for determining the position of a ball on a sports field by means of position systems, as disclosed in e.g. WO 01/66201, FR 2 753 633, FR 2 726 370, WO 99/34230, U.S. Pat. No. 4,675,816, U.S. Pat. No. 5,346,210 and WO 98/37932. These positioning systems may be used e.g. for determining if the ball has passed the border of the playing field and the positions of the players as well and provides many useful information to the referee. However, the determination of the passage of the goal plane is a very delicate matter, both because it may be decisive for the outcome of the sports match and because the distances are small and the velocity of the object often very high, so that a position determining system to provide a reliable determination of whether the object has passed the goal plane must be very precise in the determination of the position and at the same time have a very high update rate of the position determination. The object may e.g. move with 72 km/h or up to 130 km/h, which equals 20 m/s and 36 m/s, respectively, which means that an update rate of 1/100s will add an uncertainty of 20 cm or up to 36 cm, respectively, to the determined position, which is unacceptable with respect to determination of a goal in a sports match.
Position systems with a sufficiently precise determination of the position of a sports object and a sufficiently high update rate to provide reliable indications of the crossing of a goal plane, are very expensive to install and maintain. It is therefore desirable to provide an alternative system with a sufficient spatial as well as temporal resolution to provide reliable indications.
U.S. Pat. No. 5,976,038 discloses an apparatus for providing an output indication when a playing object crosses the play determinative line. The apparatus comprises a directional receiving antenna, such as a disk-reflector antenna and in particular a cassegrain antenna provided with dual, horizontally adjacent feeds, which are combined to provide sum and difference signals. The antenna is arranged outside the playing field and is directed along the play determinative line. In order to provide a sufficiently high spatial resolution due to the distance between the antenna and the playing object, the reflector of the antenna must have considerable dimensions. A reflector of 30 inch width, 76 cm, will provide a detection zone of 4 inch width, 10 cm, which together with other uncertainties of the system is acceptable for use with American football as the patent is directed at, but is unacceptable for many other sports games and a much larger reflector would be required.
U.S. Pat. No. 4,375,289 discloses two electrical conductors or emitter coils encircling or enclosing the goal plane in two vertical levels with a mutual distance in the direction perpendicular to the goal plane and emitting each an electromagnetic field by providing the two conductors with alternating current in counter-phase, so that the electromagnetic field perceivable at the object when passing the goal plane is zero at the mid-plane between the two levels due to destructive interference, and the passage of this mid-plane is determined from measurements of the field intensity at a sensor in the ball. The ball sensor employed is a passive unit that receives power from the electromagnetic field by induction of current in a coil or antennae of the sensor, and emits a signal accordingly, which is detected by a detection coil situated between the two conductors, and the direction of the passage may be detected as well by means of a phase comparison between a signal received from the ball sensor and the phases of the currents in the conductors. The system may also be designed reversely with respect to emitter and detection coils, so that one emitter coil is situated in the goal plane between two detection coils with corresponding operation of the system, so that the ball is detected to pass the goal plane when the detected signals in the two detection coils are equal.
However, this arrangement has the drawback that the spatial resolution is limited by the size of the ball as the coil of the sensor substantially encircles the ball diameter, which is of increasing importance with decreasing distance between the ball and the detection coil. This is not a major problem when detecting most scored goals when the ball clearly passes the goal plane, but in situations of doubt where the ball only just passes or do not pass the goal plane completely and the ball is close to the coils, the spatial resolution is not sufficient to decide with a satisfactory precision whether or not the goal has been scored.